Liver disease caused by infection with Hepatitis C Virus (HCV) is a leading cause of death in HIV-infected persons, and is of particular importance among injection drug users (IDUs). Several cofactors worsen HCV progression, including HIV, age, and enhanced HCV replication. Since HIV and aging impact on each other and on HCV replication, demonstrating their independent contributions to liver disease progression has been challenging up till now. This proposal is conceived to investigate the role and mechanism(s) underlying liver disease progression in HCV-infected IDUs using innovative and complementary tools in the clinic and lab. In the first aim, the effect of HIV on aging in the liver will be studed using structured longitudinal measurements of liver stiffness in a cohort of HIV-infected and - uninfected persons, and especially on the effect of suppressive HAART on fibrosis progression rates. Nested mechanistic studies are planned to understand how HIV contributes to the aging liver. The second aim, focused on HCV replication, leverages advances in tissue imaging and specimen collection to study single hepatocytes from archived HCV-infected human biopsies. Absent a representative model of human HCV infection, molecular signatures of HCV- infected hepatocytes will be compared within the same person to those of uninfected hepatocytes, identifying potential therapeutic targets for HCV control. The third aim is centered on understanding HCV replication in the context of the transition of HCV infection from treatment-responsive during acute infection to treatment-resistant during chronic infection, extending the observation that the IL28B gene locus strongly predicts treatment responsiveness in chronic infection. Persons with favorable and unfavorable IL28B genotypes will be followed during acute infection and early chronic infection with structured measurements of immune activation and tolerance in peripheral blood mononuclear cells and liver tissue to target mechanisms underlying treatment resistance. The proposed studies will define key pathways of liver disease progression in HCV-infected IDUs. The overall long-term public health impact of a successful proposal is to a) attenuate the progression of liver disease in IDUs and b) to identify relevant molecular targets for HCV control.